It has been found that brake pressure regulators which have been in use in the past employ electrically-controlled solenoid inlet and discharge valves, which were always completely closed and completely opened by the control signals, in order to maintain the pressure at the desired value. This adversely affects the precision of the regulation, and the amount of air consumption is relatively high when used in compressed air brake systems. Further, the life-expectancy of the solenoid valves is reduced by the high number of switching actuations.
A brake pressure regulator for pneumatic vehicle brakes, which avoids the necessity of completely opening and closing the solenoid valves, is shown and disclosed in published German patent application DE-OS 29 02 337. This latter type of regulator is designed with the possibility of modulating the pulse width of the control signals for the solenoid valves by superimposing onto the signal of a set value pressure. The set value pressure signal is compared to a signal, which is comparable to a measured actual value pressure, to produce an alternating voltage. In this manner, a gradual reduction of the pulse width repetition rate to 0.5 with deviation of 0 is obtained.
The fact that it is practically impossible to cause complete actuation of the solenoid valves at low frequencies, is disadvantageous. It results in great deviations and short downtimes for the electromagnets. With high-frequency alternating voltage signals, the solenoid valves are only opened very little with small deviations, and the precise adjustment requires many pulse cycles. The timing of the control cannot be controlled with accuracy. The valve would have to be completely closed, before the solenoid valve can be opened sensitively (also in small steps) by exactly measured timing of the control. Another disadvantage lies in the dependence on influences such as coil temperature, voltage, process tolerances, etc.
A literary paper on "Proportional Pressure Regulating Valves NG 4", published by the Herion-Werke KG Company, discloses a proportional pressure-regulating valve (which is known to the art) in which the secondary pressure is adjusted by a change in the magnetic force which causes a displacement of the piston. The size of this displacement is given as a voltage signal by an electrical set value. The signal is sent to the input side of an amplifier. The output current is obtained, proportional to the input voltage, at the output side of the amplifier. When sent to the magnet, this current results in a pneumatic output signal proportional to the magnetic force. Manufacture of this known pressure-regulating valve, in large numbers, is too expensive, due to the required very-precise alignments. In addition, this known valve does not operate with enough precision, since no pressure sensor has been provided. The valve is also highly dependent on outside influences.